Field of the Invention
Various types of modified starches have been used in treating paper in order to improve its strength and surface characteristics. For example, modified starch additives are commonly coated onto formed cellulosic sheets in size presses and other types of coating apparatus.
In these times of increasing awareness of the environmental aspects of manufacturing processes, it has been noted that cationic starches reduce BOD pollution problems. Cationic starches help retain fiber fines. They exhibit superior absorbancy onto anionic cellulosic fibers and onto common pigments. This improved absorbance and attendant superior pigment binding efficiency means that lesser amounts of cationic starch can be used to meet sheet specifications. Since lesser amounts of starch are used and a greater percentage of the starch used is bound onto the cellulose fiber, lesser amounts of BOD-producing starch will be present in effluents produced during paper manufacture.
These important BOD-related properties of cationic starches are believed to be due to attraction between the starch and both anionically charged pigment and fiber resulting in improved chemical anchoring of coatings to the cellulose. When cationic starch is used as a surface size, more starch is retained on or near the surface of the sheet than is retained with conventional starches. Thus, less starch is needed to maintain surface strength and quality. Since the cationic starch is tightly bound to the paper fiber, it is not removed during the repulping of broke. Hence, starch from the broke is recovered in the papermaking operation rather than being lost in the mill effluent. This attachment to the fiber coupled with the fact that reduced amounts of cationic starch are needed at the size press results in significant reductions in mill effluent BOD.
In addition, cationic surface sizing results in improved printing properties. These improved properties are believed to be due to a combination of fiber bonding and surface orientation resulting in more uniform starch concentration on the paper surface, and hence more uniform ink receptivity and improved ink hold-out. The improved fiber bonding provides a strong paper surface. Among the various improved printing characteristics experienced with the use of cationic starches are: better printing uniformity, better print definition, greater depth of color, reduced ink show-through, fewer pick-outs on offset press and reduced dusting.
Cationic starches are generally manufactured in an approximately 12 hour process by reacting the granular starch in an alkaline medium with appropriate tertiary or quaternary amines, followed by neutralizing, washing and drying. The alkaline agent serves to promote reaction of the amine with the starch. Thus, in production of such commercial dry cationic starches, a special manufacturing process is required, increasong cost and offerring potential stream pollution hazards.
U.S. Pat. No. 4,029,885 describes a means for avoiding problems associated with the use of the alkaline agent which entails the treatment of gelatinous anionic starches with a cationizing agent. The anionic starches employed in U.S. Pat. No. 4,029,885 include commercially available oxidized starches and ammonium persulfate process pearl starches.
A method for producing cationic starches which has the important advantages of the U.S. Pat. No. 4,029,885 starches and also exhibit significantly enhanced retention on the paper fiber has now been discovered. As elaborated further below, this method requires the use of a limited group of anionic starches and the use of a very specific cationizing procedure.